Have you tried cutting down his 2:30-4pm nap by a half hour or even 15mins?. The reason i'm asking is that i know if my girls aren't awake for more than 3 hours, they are still wide awake. Your child maybe well rested and perhaps that's why he can't sleep yet.
Sometimes after the nap, i'd feed my babies, they play to poop themselves a little and then we'd do our winddown routine and in bed. We're usually off bedtime by 15-30mins but i figure EASY isn't about 'the clock' but the routine itself.